1. Field of the Invention
The present invention relates to an ion doping apparatus and a doping method using the same, and in particular, a high-precise ion doping technology using impurity regions of source and drain regions of a thin film transistor (TFT), and so on.
2. Description of the Related Art
The technology for ionizing impurity elements used for the control of valency electrons of a semiconductor and accelerating the ionized electrons in the electric field for injection has been known as an ion injection method. In late years, the doping has been performed by irradiating ions like a shower for injecting impurity elements into a large area substrate of a liquid display device, a light emitting device, or the like.
The ion doping apparatus (hereinafter, also simply referred to as a doping apparatus) is designed such that a doping chamber is communicated with an ion source and is kept under vacuum while placing a substrate therein to subject the surface of the substrate to an ion current irradiated from the ion source. The ion source comprises a plasma chamber, a lead accelerating electrode system for pulling out ions generated in the plasma chamber, and a decelerating electrode system for controlling the influx of secondary electrons. In this case, a porous electrode is generally used as an electrode, so that ions pass through the pores to form an ion current directing toward the doping chamber.
As a method for plasma generation in the ionic source, there are several processes known in the art, such as a direct discharge system, a high frequency discharge system, and a microwave discharge. In addition, plasma can be confined in the inside of the ion source by the application of an electric field. Alternatively, a cusp magnetic field may be formed by arranging a permanent magnet around the plasma chamber.
In many cases, such a doping apparatus does not require a mass separation, so that all of ion species (positive charges) formed in the plasma chamber is accelerated in the electric field caused by the lead electrodes and injected into the substrate. In many cases, material gas (diboron (B2H6) or phosphine (PH3)) diluted with dilution gas such as hydrogen is used as a material of gas for the generation of ions. As a result, in addition to the objected impurity ions (boron ions and phosphorus ions), a large amount of hydrogen ions is introduced at the same time.
In the case of using diboron as material gas, ions such as H+, H2+, H3+, BHx+ (X: 1-3), B2Hy+ (y: 1-6) can be generated. The abundance ratio of these ion species depends on the dilution ratio of material gas and the conditions of plasma generation. When accelerating in the electric field without mass separation, a plurality of these ion species will be irradiated on the substrate.
Concretely, a spectrum shown in FIG. 9 can be obtained by the measurement with an EXB mass separator on each of ion species generated at the time of using diboron gas diluted to 5% with hydrogen as material gas. In this case, the peak of B2Hy+ ion is observed in the vicinity of a mass number of 20. Furthermore, the peak of H+ ion at a mass number of 1 and the peak of H3+ ion at a mass number of 3 are observed, respectively.
A faraday cup electrometer (FCE) is used as the doping apparatus and monitors an ion current for adjusting the dose amount of the doping. However, the FCE measures only a current value based on the total ions including diluted-gas ions generated from the diluted gas in addition to the impurity ions such as phospine and diboron (used for the control of valency electrons) generated from the material gas. Therefore, there is a problem that the amount of impurity ions to be injected changes as the ratio of the respective ions generated in the plasma chamber changes.
FIG. 10 is a graph that represents the distributions of elements (boron) with the respective mass numbers of 10 and 11 in an oxidative silicon membrane in the depth direction, which is measured by a secondary ion mass spectrometer (SIMS). The figure shows the changes in concentration at the time of sequentially doping a plurality of substrates using the doping apparatus. In the data shown in the figure, the concentration of boron increases as the number of the doping treatments increases (i.e., the doping process proceeds in the later half) even though each doping treatment is set to the same dose amount. Therefore, the results indicate that the number of the doping treatments increases as the ratio of boron-containing ion species increases.
In addition, in FIG. 11, there is shown variations of threshold voltages among the substrates of TFTs prepared by performing the channel doping under the same conditions. In this case, also, it is observed that the threshold voltage tends to shift to the plus side as the number of doping treatments increases (the number of substrates being processed with doping increases). The results indicate that the amount of introduction of boron increases.
Furthermore, in one of the prior art documents (e.g., Japanese Laid-Open Patent Application No. 2001-357813), there is a method for independently measuring each of the ion species generated from the material gas including the diluted gas by polarizing and separating these ion species with a polarizer. Therefore, such a method allows the control of the doping amount of each ion.
However, several problems have been found in the above process comprising the steps of separating each ion, measuring the concentration of the ion from a current value based on each ion to adjust the amount of the doping. For example, when the amount of the objective ion species such as channel dope is low, the objective impurity ion cannot be detected as the concentration thereof becomes lower than the lower limit of the detectable range.